Induction machines are well known; however for a long time it has been considered that synchronous machines were much better for generating alternating current, and the induction generator has been considered less commercially significant and its importance for power generation downplayed.
When variable speed energy sources are operating independently of other energy systems, electricity of reliable and usable frequency and current can be obtained effectively and cheaply by utilising relatively simple and low cost power electronic controls. However, when variable speed energy sources are to be integrated with large scale supplies and connected to large scale grids, sophisticated and expensive electronic controls are required. This is because power grids demand power input of a selected active and reactive power characteristic; that is, the voltage and current wave forms must be phase related in a selected manner to meet the requirements of the connected load.
To provide this integration when using conventional induction generators, and since singly fed induction generators are fundamentally incapable of supplying reactive power, such a generator must be doubly-fed—that is it must have a second full power input, usually and most commonly in the form of a pulse width modulated converter connected to its (wound) rotor circuit.
Such a doubly-fed machine requires brushes and sliprings. There are very significant maintenance costs and problems associated with brushes and sliprings for a generator which is located at the top of a high tower, where such a generator is usually located in order to generate maximum power from a large rotating wind-driven propeller or prime mover. (Another form of a doubly-fed machine which is not generally satisfactory in a power generation application has TWO stator windings which interact with each other and upon the conductors of a common rotor winding.) Induction generators are usually coupled to the prime mover by means of a full speed reduction gearbox, since such a generator does not require as many poles as one without a gearbox. However a full reduction gearbox adds a very significant mass to the system, which is undesirable in terms of cost and logistics when located at the top of a high tower.
Alternatively conventional synchronous generators may be used, however these are fundamentally constant speed devices and can only be used in wind applications if the output is rectified and then converted back to mains frequency. Whilst this eliminates the necessity for a gear box, the high cost of a full power (PWM) inverter makes conventional synchronous generators very expensive. Brushes and sliprings can only be avoided by using permanent magnet rotor excitation along with some other means to regulate or control the output voltage.
Brushless twin stator squirrel cage induction generators (BTSIG) are known as being suitable for generation in variable speed input situations, a particular version described by Edwin Sweo, wherein first and second generating machines are connected at their rotors by connecting the conductors in the first cage rotor to the conductors in the second cage rotor in a reverse phase sequence. This system introduces assembly and mechanical difficulties.
However, such a machine does not of itself constitute a substitute for other forms of variable speed generators, including the doubly-fed induction generator and the permanent magnet synchronous generator used in conjunction with a full power inverter referred to above.
The present invention seeks to alleviate one or more of the disadvantages of the generator arrangements referred to above.